1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method, and specifically relates to a configuration for reducing density unevenness that occurs when printing is performed by reciprocal, bidirectional scanning.
2. Description of the Related Art
Personal computers, word processors, and other office automation devices have come to be used widely in recent years, and various printing apparatuses are provided for printing out information processed by such devices. As a trend of such printing apparatuses, high image quality and high speed are being demanded, and various techniques for these purposes are provided.
High Quality Image Printing Technologies
As an example of a high image quality technology, a so-called multi-scan method is known. With this method, scanning of a print head is performed a plurality of times on a same region and in the plurality of scans, different ink ejection ports are set to be used to perform printing.
In a case of performing printing using a print head that is provided with a plurality of printing elements, such as ink ejection ports, etc., a quality of a printed image is largely dependent on a precision of the print head. In a print head manufacturing process, for example, fluctuations may arise in shapes of the ejection ports of the print head or in a set position of an ejection heater for generating energy for ejection. Such fluctuations become apparent as slight differences in ejection amount, ejection direction, and other ejection characteristics among the plurality of ejection ports in the print head and, cause density unevenness in an image finally formed to decrease the grade of the image.
FIGS. 1A to 1C and FIGS. 2A to 2C are diagrams for explaining specific examples of the above-described degradation of the image. In FIG. 1A, a reference sign 101 schematically denotes a print head, and for simplification of description, this is illustrated as having eight ink ejection ports 102. A reference sign 103 denotes ink droplets ejected from the respective ejection ports 102, and normally, it is assumed that ink is ejected at a substantially same ejection amount and in a same ejection direction as shown in the figure. By such ejection, dots of a substantially same size are formed on a paper surface as shown in FIG. 1B and a uniform image without density unevenness as a printing image as a whole is obtained (see FIG. 1C).
However, as mentioned above, in actuality, there are fluctuations in the respective ejection characteristics of the plurality of ejection ports in many cases. As a result, fluctuations may occur in the sizes and directions of ink droplets ejected from the respective ejection ports as shown in FIG. 2A. Consequently, dots that differ in position and size may be formed as shown in FIG. 2B. In this case, a blank portion, in which an area factor of 100% is not attained, or oppositely, a portion, in which dots are overlapped more than necessary, may arise in a cyclical manner and a white streak or black streak may form as shown in a central portion of the figure. An image formed with a set of dots having such state has a density distribution in an ejection port array direction such as shown in FIG. 2C and this is consequently perceived as density unevenness.
A multi-scan method resolves such a problem of density unevenness to improve the image quality. FIGS. 3A to 3C and FIGS. 4A to 4C are diagrams for explaining this method. In the multi-scan method, a plurality of scans, that is, in the example shown in FIG. 3A, two scans of a print head are performed to complete printing of a predetermined area (in the examples shown in these figures, a 100% duty printing of forming dots respectively in all pixels in the predetermined are). More specifically, an area (corresponding to four pixels) of half the area shown in FIG. 1B or the like is completed by two scans (herein after, also referred to as “two passes”) between which a printing medium is conveyed by an amount corresponding to the area corresponding to four pixels. In this case, eight ink ejection ports 202 of a head 201 are divided into a group of four upper ejection ports and a group of four lower ejection ports. In addition to the above, the dots formed by the ink ejected from a single ejection port in a single scan are thinned, for example, by half in a scan direction array by using a mask. A remaining half of the dots are then formed in a second scan using a mask that complements the aforementioned mask to complete printing of the area corresponding to four pixels. FIGS. 4A to 4C are diagrams of an example of a mask and a dot pattern formed using the mask. The mask and the dot pattern shown in these drawings are of a checker pattern, which is the simplest pattern according to which dots can be formed vertically and horizontally in each pixel. The printing is completed by a first scan (FIG. 4A or 4C), by which the checker pattern is printed in a unit print area (corresponding to four pixels in the present case), and a second scan (FIG. 4B) of printing a complementary checker pattern.
With the above-described multi-scan method, even when the print head having the ejection characteristics shown in FIG. 2A is used, the influence of the ejection characteristics of the respective ejection ports can be reduced to ½ and the printed image becomes as shown in FIG. 3B. White streaks or black streaks can thereby be made less conspicuous. Consequently as shown in FIG. 3C, the density unevenness also is decreased in comparison to the case shown in FIG. 2C.
High Speed Printing Technologies
At the same time, a bidirectional printing is known as an example of a high speed printing technology. This printing method is a method in which, in a serial type printing apparatus, after performing printing by a forward direction scan of a print head, conveying a paper by a predetermined amount is performed, and a printing scan is also performed in a subsequent movement of the print head in a backward direction. With this printing method, in comparison to unidirectional printing, in which printing is performed in the forward direction scan but printing is not performed in the returning movement of the print head in the backward direction, double the printing speed or the throughput, by simple calculation, can be achieved.
The bidirectional printing can be used in both so-called one-pass printing, in which printing of a scan area having a length corresponding to an ejection port arrangement width of a print head is completed in a single scan of the print head, and the above-described multi-scan printing, in which printing of the scan area is completed with a plurality of scans between which a paper conveyance is performed. Thus by performing bidirectional printing with use of the multi-scan method, both high quality image printing and high speed printing can be realized.
However, it is also known that when bidirectional printing is performed with use of the multi-scan method, density unevenness (time interval unevenness) occurs due to a difference of a time intervals in the plurality of scans, between positions in a scan area.
FIG. 5 is a diagram for illustrating the time interval unevenness and shows an example where printing is completed in two scans (passes) in opposite directions to each other. In FIG. 5, when focusing attention on left and right end regions of a printing medium, a region in which a second printing is performed immediately after a first printing, and a region in which, after a first printing, a second printing is performed on elapse of a scan time corresponding substantially to two scans of forward or backward scan, appear alternately. As a result, at the left end region, a printing area 1 has high image density and a printing area 2 has low image density, and this density difference appears alternately. At the right end region, the printing area 1 has low image density and the printing area 2 has high in image density, and as in the left end, this density difference appears alternately. Also in each printing area, the density varies along a scan direction. For example, in the printing area 1, a left end side is high in density and the density decreases toward the right end side.
The above described phenomenon occurs due to differences in a time during which a precedently landing ink droplet permeates into an interior of a printing medium and becomes adsorbed into paper fibers or an ink receiving layer, etc., and then landing of a subsequent ink droplet is performed. If there is sufficient time for adsorption of the precedently landing ink into the paper fibers or the ink receiving layer, the subsequently landing ink droplet permeates gradually in a direction of gravity while seeking a portion into which it can become adsorbed comparatively smoothly. On the other hand, if there is not enough time for the precedently landing ink to become adsorbed into the paper fibers or the ink receiving layer, the subsequently landing ink droplet joins the precedently landing ink and permeates gradually in the gravity direction as a single aggregate of ink droplets. In the latter case, the precedently landing ink joins the subsequently landing ink before becoming adequately adsorbed by near a paper surface and becomes adsorbed at a lower portion. Consequently, the image density becomes comparatively low. In a case where printing of a secondary color is performed, the density unevenness appears as a color unevenness corresponding to the scan time intervals.
FIGS. 6A and 6B are diagrams for illustrating an occurrence of density unevenness in accordance with such ejection time intervals of ink droplets. A case where the ejection time interval of two ink droplets is long is illustrated in FIG. 6A, and a case where the ejection time interval of two ink droplets is short is illustrated in FIG. 6B. In FIG. 6A, a precedently ejected ink droplet lands on a printing medium and permeates and becomes fixed in an interior of the printing medium. After fixing takes place over a comparatively long time, a subsequently ejected ink droplet lands, permeates so as to get into under the precedently ejected ink, and becomes fixed below the precedently ejected ink. On the other hand, in the case shown in FIG. 6B, the precedently ejected ink droplet lands the printing medium and permeates into the interior of the printing medium. In this case, since the time until the subsequent ink droplet lands is short, the subsequently ejected ink droplet lands while the precedently ejected ink droplet is in the process of becoming fixed. Therefore, the unfixed ink of the precedently ejected ink droplet and the subsequently ejected ink droplet thus permeate as a single body of ink and becomes fixed finally. As result, the precedently ejected ink droplet is pushed downward by the subsequently ejected ink, the extent of permeation becomes lower below the paper surface and the density thus becomes lower. That is, as shown in FIGS. 6A and 6B, the depth of permeation of the precedently ejected ink droplet, in other words, the fixing position of the precedently ejected ink droplet differs, and the higher this position, the higher the image density. That is, the longer the time interval, the higher the density.
As explained above, in accordance with the difference in time intervals between a plurality of times of printing for performing printing on one region, a density difference occurs between the left end and the right end regions of a unit area for which printing is completed in the plurality of times of scan. Also as shown in FIG. 5, when such unit areas are adjacent to each other, regions of different density become adjacent alternately and this becomes recognized as a density unevenness (herein after, also referred to as “between-band unevenness”). In particular, the density difference is large and the between-band unevenness can be recognized conspicuously at the left and right end regions of the unit area.
As a technology for suppressing such time interval unevenness caused by the time difference of printing, there is a technology described in Japanese Patent Laid-Open No. 2003-34021. In this document is described a switching of a printing mode from bidirectional printing to unidirectional printing when a possibility of occurrence of density unevenness is high. Specifically, a printing area is divided into a plurality of areas in a main scan direction, and numbers of dots of black ink and color ink to be applied to each area are counted. When there are areas, in which threshold values are exceed for both black and color inks, the number of such areas is counted, and when this number of areas is no less than a predetermined number, it is deemed that the possibility of occurrence of time interval unevenness is high and switching to unidirectional printing is performed. The time interval unevenness that occurs at opposite end regions of a printed image can thus be suppressed. It is also described in the document that a width of printed image data, that is, a width of a range in which a print head scans is detected and when the width is small, it is deemed that a degree of time interval unevenness is small and switching to unidirectional printing is avoided even when the number of areas is no less than the predetermined number.
As another technology for suppressing time interval unevenness, Japanese Patent Laid-Open No. 2005-144868 describes that by making the number of passes of multi-scan increased when a printing width is long, the time interval unevenness between bands can be made inconspicuous. It is also described in the same document that the time interval unevenness can be made less recognizable by making the time interval unevenness be repeated at a high frequency. It is also described that when the printing width is long, the time interval unevenness can be reduced by raising a scan speed of a print head to shorten time intervals among multiple passes or by switching to unidirectional printing to make a printing time in each band the same.
However, in the case where switching to unidirectional printing is performed when the possibility of occurrence of density unevenness is high as described in Japanese Patent Laid-Open No. 2003-34021 and Japanese Patent Laid-Open No. 2005-144868, the significance of bidirectional printing, which is employed to achieve high speed of printing, becomes lost. Further, increasing the number of passes of multi-scan leads to lowering of an overall throughput. The change of scan speed requires a change of printing resolution.
Conventional methods of resolving time interval unevenness thus accompany comparatively large changes in printing operation or process details and cause various problems such as those mentioned above.